`rustc_splat` is not feature gated in functions without bodies

[JonathanBrouwer]

I tried this code:

fn help(x: fn(#[rustc_splat(this = completely invalid $$$)] usize)) {

}

I expected to see this happen: compilation fails because rustc_splat is unstable

Instead, this happened: the code compiles

Meta

rustc --version --verbose:

rustc 1.98.0 (88d9e12ae 2026-08-18)
binary: rustc
commit-hash: 88d9e12ae178fab0fb5cc050a94da85685d449ea
commit-date: 2026-08-18
host: x86_64-unknown-linux-gnu
release: 1.98.0
LLVM version: 22.1.8

[JonathanBrouwer]

@rustbot claim

[JonathanBrouwer]

cc @teor2345
Should rustc_splat be allowed on fn ptrs?

[JonathanBrouwer]

Also for all functions without bodies, such as in traits:

trait Test {
    fn help(#[rustc_splat(invalid = fine $$$)] x: usize);
}

[teor2345]

cc @teor2345 Should rustc_splat be allowed on fn ptrs?

Edit:

Also for all functions without bodies

Yes for both, see #159643. This is important for use cases which involve overloaded function types (rather than function items).

This attribute is meant to be feature-gated, really not sure how it escaped containment.

Note that this is technically usable on stable, without invoking the splatted function item feature gate:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=3cc5a899f831df45145f0f71884ee86b

[teor2345]

Minimised:

fn help(x: fn(#[rustc_splat] usize)) {}
trait Test {
    fn help(#[rustc_splat] x: usize);
}

Note that rustfmt stable and beta deletes this attribute, this was fixed in nightly in #161709